Measuring Spatial Access to Resources at Scale
St. Albans MLK Day of Reflection
January 20, 2026
Gabe Morrison
Senior Data Scientist
The Urban Institute
Who am I
- Graduated from Brophy College Preparatory in Phoenix, AZ
- Studied a mix of Geography, Public Policy, and Computer Science
- Data scientist at the Urban Institute:
- Build tools to support communities’ support fair resource allocation and upward mobility from poverty
Run of Show
- Motivation: Why is spatial access important
- Present on some spatial and demographic data analysis concepts
- Break you all in three groups to have you all consider a spatial access measure
- We will share out to the group
- Real life example
- I will present a bit on how we think about these things in practice for a piece of software I have co-built and work on at Urban
Motivation
- Government resource allocation: Identify disparities or suitable opportunities for investment
- Business (location intelligence): Determine underserved markets
- Assess property values: Proximity to amenities affects real estate pricing
Key Concepts: Point and Polygon Data
Point Data
- Used to represent a single location
- Examples:
- Location of post offices
- Bus stops
- Libraries
Polygon Data
- Used to represent a shape or area
- Examples:
- State borders
- Census tracts
- School districts
Census Tracts
Definition: Census tracts are small, relatively permanent statistical subdivisions of a county defined by the U.S. Census Bureau. They typically contain between 1,200 and 8,000 people, with an optimal size of 4,000 people.
- Often closely aligned with a concept of a “neighborhood” in cities
- Rural areas: they are much larger (to encompass enough population)
- Boundaries generally follow visible features (roads, rivers) and governmental boundaries
Census Tracts: Urban vs. Rural
Centroids
- Informally: The center of a polygon
- More formally: The geometric center or “average location” of all points in a polygon
Spatial Access Methods: Group Activity
- Break into three groups
- Each group reviews one method on the following slides
- Your tasks:
- Review and understand the approach
- Determine the strengths and limitations of the approach
- Identify an example for when the method would be suitable and unsuitable
- Nominate a group representative to share out your results
Method 1: The Container Method
Steps:
- Count the number of points (resources) in a given census tract
- Treat all points in the polygon as accessible to residents of the census tract
- Treat all points outside the census tract as inaccessible
- Optionally, divide number of points by the population of the tract to get a “resource per person” measure
Method 2: Minimum Distance
Steps:
- For each tract, start with a tract centroid
- Calculate the minimum distance from the centroid to a given resource
- The closer the nearest resource, the higher the access
Method 3: Gravity Potential
Steps:
- Start with the tract centroid
- For the tract centroid:
- Calculate the distance to each resource
- Divide 1 by the square of each distance
- Sum those values
Formula: \(access_i = \sum_{j}{\frac{1}{distance_{ij}^2}}\)
Discussion Questions
For your assigned method, discuss:
- What are the strengths?
- What are the limitations?
- Provide a use case that demonstrates the strength
- Provide a use case where the measure is inappropriate
Thank You
Questions? Ask me anything!
Takeaway: When you travel from your home to some other place, think about the right way to measure how easy it is to get there